1. Field of the Invention
The present invention relates to a process for preparing a radiation-sensitive composition containing a fractionated polymer, wherein the fractionated polymer is formed by passing a crude polymer mixture through a porous polymeric media. The present invention also relates to radiation-sensitive compositions containing fractionated polymers made by the above process.
2. Brief Description of Art
As integrated circuits (IC) devices become higher in memory density, photolithographic image patterns formed on wafers are required with less than 0.3 micron (.mu.) in resolution. To achieve this objective, photoresists used for photolithographic imaging are required to have a higher resolution, faster sensitivity, and more vertical profile than the state-of-the-art photoresists of a decade ago.
Most common g/i-line positive working-type photoresists currently used in IC device manufacture consist of an alkaline-soluble polymer (e.g., novolak resin, polyhydroxystyrene resin, copolymers of hydroxystyrene and styrene/polyhydroxystyrene resin, and the like) as a binder resin, and a photoactive compound (PAC) such as a naphthoquinonediazide (DNQ) ester of a polyphenol compound. Preferred binder resins useful for the g/i-line applications, are selected from the group of novolak resins which are prepared by addition-condensation reaction of various phenolic derivatives with various aldehydes or ketones in the presence of acids such as oxalic acid, sulfuric acid, and the like. Among various phenolic derivatives used as a monomer of the novolak resins, meta-cresol and para-cresol are some of the most useful monomers. For example, combinations of novolak-type phenol resins and DNQ esters as a positive-working photoresist are disclosed in U.S. Pat. Nos. 5,547,814 and 5,407,779.
For high-resolution formulations, chemically amplified photoresists have been utilized instead of the novolak/DNQ combinations. These chemically amplified photoresists generally utilize a combination of a selected polymer resin such as a partially modified or protected poly(hydroxystyrene) resin or copolymer of a partially modified hydroxystyrene with other monomers such as acrylates or methacrylates, a photoacid generating compound (PAG), and a selected solvent.
For both novolak/DNQ and chemical amplified formulations, control of the structure of the polymer matrix of a photoresist is crucial for the superior lithographic properties which are required from modern integrated circuits. Conventionally, fractionation of polymers used in photoresists was carried out by dissolving the polymer in a known solvent and precipitating it with a calculated amount of a non-solvent. The fractionated polymer may then be isolated from the remaining solubilized polymers. However, this solvent fractionation process generates large amounts of solvent wastes that can be difficult to dispose of. The issue of solvent waste has been addressed by employing supercritical liquid carbon dioxide instead to fractionate these types of polymers. However, this approach has been hindered by high cost and unreliable scalability.
The above approaches also suffer additional disadvantages. Solubility of the polymers is dependent on a multitude of thermodynamic factors, notably temperature, and other physical parameters. Strict process control of these parameters is necessary to obtain fractionated polymers with the desired molecular weight range. Process control of this kind often requires complex, specialized equipment that adds to the cost of producing the products. Differential solubility approaches to polymer fractionation may result in a high degree of polydispersity of the fractionated polymers. Accordingly, in many cases, a better fractionation process is needed to produce a more homogeneous population of polymers (i.e., lower polydispersity) without the disadvantages of the prior art fractionation methods.
Thus, what is needed in the photoresist art is a reproducible, low-cost process of fractionating polymers for use in photoresists that results in substantially homogeneous polymers of a selected molecular weight. The present invention is believed to be an answer to that need.
Separately, Pall Corporation (East Hills, N.Y.) manufactures a series of ultrafiltration modules using membranes made from hollow fibers that are specifically designed for use in biotechnology and pharmaceutical industries. These ultrafiltration modules are designed to purify and concentrate diverse molecules such as protein-based pharmaceuticals, amino acids, antibiotics, low molecular weight products from fermented broths, and the like. However, such ultrafiltration modules or membranes have not been used to purify polymer resins for use in photoresists or other radiation-sensitive compositions used in semiconductor fabrication processes.